Various technologies have been used in time division switching systems as interconnection facilities so that the system port circuits can communicate with one another during the serving of calls. These facilities increase in complexity as the size of the system increases. The J. C. Moran U.S. Pat. No. 3,996,566, issued Dec. 7, 1976, discloses a switching system having a single PAM type time division bus connected to all of the system port circuits. Two port circuits are interconnected on a call by assigning them to the same time slot and by activating circuitry within the two port circuits which permits them to exchange speech samples over the bus during each occurrence of the assigned time slot.
Single bus systems as shown by Moran are suitable only for use in small to medium size installations since the system's call serving capacity is limited by a number of factors including the number of system time slots. For example, in a 64-time slot PAM system, no more than 64 simultaneous calls can be served--regardless of the number of lines and/or trunk port circuits provided. The system's call serving capability cannot easily be increased by merely increasing the number of time slots since this presents other problems--such as increased costs of the port circuit sampling circuitry. Thus, there are limits imposed by economical and technical considerations regarding the number of simultaneous calls that can be served by single bus systems of the type shown by Moran.
Increased call serving capacity has been provided in prior art time division switching systems by the use of switching facilities of greater complexity. One such prior art arrangement, which is useful for medium size systems, is shown in the article entitled "The GTD-100 Digital PBX" in the AUTOMATIC ELECTRIC JOURNAL of March 1977, pages 262 through 268. The system shown on FIG. 6 of this article comprises a plurality of groups of port circuits with each group being sampled at a 24-time slot rate. The samples from the various port circuit groups are combined by multiplexing them first up to a 96-time slot signal and then up to a 192-time slot signal. The call switching and time slot interchange functions are performed at the 192-time slot stage. This upward multiplexing permits the system to provide a greater call serving capacity than the Moran system; however, it does so at the price of increased cost and complexity.
Further increases in call serving capacity have been provided by arrangements of still further complexity, such as by the use of networks of the time-space-time type. This is shown on FIG. 5 of the article entitled "New Digital Electronic PABX Family", pages 303 through 311 of the AUTOMATIC ELECTRIC JOURNAL of May 1977. This figure shows a system having a time-space-time network in which a group of port circuits is sampled at a 24-time slot rate. The signals from the various groups are combined by multiplexing them up to a 96-time slot signal and then up to a 384-time slot signal which is applied to a time slot driven space division switch. This switch selectively interconnects the various 384-time slot signal paths to perform its call serving functions. The network shown in FIG. 5 of this article is shown in further detail in FIG. 1 of the article entitled "GTD-4600 Network Description" on pages 57 through 65 of the AUTOMATIC ELECTRIC JOURNAL of March 1978.
It is also known to use plural module systems in order to provide increased call serving capability. However, system timing and synchronization facilities present a particular problem in plural module time division switching systems. Great precision is required of these facilities in the serving of intermodule calls so that each call signal is sampled at the correct time in a first module, processed by the circuitry of the first module, transmitted to a second module, processed by the circuitry of the second module, and applied to the time division bus of the second module during the correct time slot.
System timing and synchronization is critical for these operations since, for quality call service, it is necessary that each call sample be applied with timewise precision to the various circuits and circuit elements involved in the processing and transmission of the sample. The call samples are not normally held in storage and are essentially in transit from the time they are generated at a first station until they are applied to and received at a second station. If the timing and synchronization circuitry does not operate with the required precision, degraded call service in the form of distortion, crosstalk, etc. will result.
Prior art plural module time division switching systems utilize arrangements in which each module has its own oscillator as well as arrangements in which the entire system has a single master oscillator that drives a clock within each module. Both of these arrangements involve the use of complex circuitry. Systems having an oscillator in each module require a master system timing source for keeping all module oscillators precisely on frequency. Systems that have only a single master oscillator require expedients such as delay lines, cables cut to the precise length, etc. in order to achieve precision frequency and time slot coordination between modules.
In summary, although prior art arrangements are known which enable plural module time division switching systems to serve whatever reasonable level of traffic may be desired, this is done at the expense of increased cost and system complexity such as the use of time-space-time networks, upward multiplexing facilities, and complex timing and network synchronization facilities.